Polyphase DC motors, and more particularly three-phase DC motors of the brushless, sensorless type, are widely used in computer system disk drives, such as floppy disk, hard disk, or CD ROM drives, as well as in other applications. Three-phase DC motors may be modeled as having a stator with three coils connected in a "Y" configuration, although typically a larger number of stator coils are employed with multiple motor poles. In conventional applications, eight pole motors are used having twelve stator windings, or coils, and four N-S magnetic sets on a rotor. The twelve stator coils are modeled in terms of four groups of coils, each group being arranged as a set of three "Y" connected coils. One end of each of the three coils in the "Y" configuration is joined to a common node, or a center tap, and the opposite end of each coil is connected between a high side driving transistor and a low side driving transistor. The center tap may be left unconnected, or it may be connected to a controlled voltage source.
A three-phase DC motor is typically operated in a bipolar mode which can be summarized as follows. The three "Y" connected coils are energized in a sequence of patterns or pathways of current to drive the rotor. In each pattern a current path is established through two of the three coils. The third coil in the "Y" configuration is left floating, or, in other words, no current is permitted to flow through the coil.
Current flow through each of the "Y" connected coils is controlled by the driving transistors. Current flows in one of the three "Y" connected coils when either its high side driving transistor or its low side driving transistor is energized to conduct the current. In each pattern current flows through a high side driving transistor and its associated coil, through the center tap, and then through a second coil and its low side driving transistor. The sequence of current pathways is chosen so that, as the current path is changed, one of the coils in the current path is switched to a floating condition, and the previously floating coil is switched into the current path. In the "Y" configuration of three coils a total of six different current paths are available to drive the rotor. A commutation occurs each time the current path through the coils is changed, and the position of the rotor at that moment is a commutation point. In the sequence defined above, six different commutation events occur for each full rotation of the rotor in the three-phase DC motor.
Precise control of the rotational motion of the rotor in the three-phase DC motor is important in disk drive systems. Inadequate control of the rotor's motion can result in unwanted vibrations and acoustic noise. The motion of the rotor is controlled by choosing the commutation points in a precise and consistent manner. An optimum commutation point is selected based on the position of the rotor, which is typically ascertained by monitoring a back EMF signal in the motor, also called the BEMF signal, which is the EMF induced in the floating coil by the rotating magnetic field of the rotor.
The BEMF signal in the floating coil is sinusoidal in nature, and crosses the voltage of the center tap at regular intervals. The BEMF signal may be used to determine the rotational speed of the rotor. As the rotor speed increases, the frequency of the BEMF signal increases. As the rotor speed decreases, the frequency of the BEMF signal decreases. The BEMF voltage signal is also used to select the commutation points. Conventionally, optimum commutation points are chosen in relation to the moments in which the BEMF signal equals the center tap voltage, and these moments are zero crossing points. When the motor is functioning properly the position of the rotor is known at each zero crossing point.
Typically, the driving transistors in a three-phase "Y" connected DC motor are n-channel DMOS transistors having a gate, a drain, and a source. The gate is a control terminal for the n-channel DMOS transistor. An n-channel DMOS driving transistor is switched ON to direct current through a coil by raising the voltage on its gate. Conversely, the driving transistor is switched OFF by reducing the voltage applied to its gate. As the n-channel DMOS driving transistor is switched ON by increasing the voltage on the gate, current begins to flow through the transistor and reaches a plateau before the voltage applied to the gate reaches a maximum. The faster the voltage on the control terminal is raised to a maximum, the faster the driving transistor is switched ON and the faster the current in the coil rises to the plateau. Conversely, when the n-channel DMOS driving transistor is being switched OFF the voltage on the gate is reduced until no current is flowing through the transistor or the coil. The faster the voltage is reduced on the gate the faster the driving transistor stops the current flow. The rate of change of voltage at the point of connection between the driving transistor and the coil is directly related to the rate of change of current through the coil, and is referred to as the slew rate. The slew rate is governed by the voltage applied to the gate of the driving transistor.
When the rotor of the DC motor is rotating at or near a desired speed and the BEMF signal has a high frequency, a moderate amount of current is circulated through the coils to apply sufficient torque to maintain the speed of the rotor. However, during a start-up procedure the rotor is rotating at a slow speed and is generating a low frequency BEMF signal. During this period, the rotor is being accelerated by the coils which draw substantially more current than is required to merely maintain the speed of the rotor. The high current through the coils, also called high current recirculation, has a negative effect on the operation of the motor, particularly during commutation events involving the low side driving transistors. When a first low side driving transistor is being switched OFF to stop current in a first coil, a second low side driving transistor is being switched ON to allow current to flow in a second coil. When the first low side driving transistor is switched OFF rapidly, acoustic noise is generated from transient disruptions in the current through the coils for the following reason. The current through the second coil increases slowly due to a slow time constant in a control loop for the coil, and when the first low side driving transistor is switched OFF rapidly, the total current through the coils decreases slightly until a maximum current is reached in the second coil. The transient decrease in current through the coils, which is of particular significance during periods of high current recirculation, causes audible noise which is highly undesirable in disk drive systems.
Careful control of the slew rate of the driving transistors reduces high frequency harmonic noise which is generated when the driving transistors are switched ON and OFF. Harmonic noise is generated over a wide band of frequencies if the driving transistors are switched ON and OFF too rapidly, and the noise may interfere with a channel transferring data to or from the disk during a write or a read operation. A reduction in the slew rate reduces the band of frequencies over which the harmonic noise is generated, thereby eliminating interference with the read or write operations.
In conventional three-phase DC motors a constant slew rate is employed regardless of the operating conditions of the motor which results in the generation of undesirable noise. A need remains for a method and an apparatus to reduce unwanted audible noise in a three-phase DC motor.